Actual bicarbonate is the concentration of hydrogen carbonate in the plasma .
The ABG machine usually reports this as cHCO3-(P). It is a derived variable.
One is interested in the bicarbonate value because it is the most important extracellular fluid buffer, accounting for 75% of total buffering in metabolic acid-base disturbances (the rest being performed by blood proteins, such as haemoglobin).
Derived and measured bicarbonate values
Yes, the ABG machine derives its value by calculating it on the basis of pH and pCO2. In contrast, the machine in the biochemistry lab usually uses an enzymatic assay, which is a "direct" measurement.
Does that mean the ABG machine is wrong?
In short, no.
Let us be clear. In the serum, at any given time, there should be only one true bicarbonate value, and it is described perfectly by the Henderson Hasselbalch equation. If one compares their lab value to the ABG machine value, one may arrive occasionally at a discrepancy which would cause one to pause, wondering which is the more accurate; in which case the answer would be whicher sample got to the analyser faster. The agreement between the two ways of measuring the bicarbonate value appears to be within 3mmol 96% of the time, and the authors who performed the comparison suggested that the discrepancy is more related to preanalytical error and idiosyncrasies in sample handling.
This has implications from a public health standpoint. Authors from countries which have less access to fancy equipment have remarked that conventional venous bicarbonate measurements are a satisfactory surrogate in a situation where an ABG analyser is not available (eg. rural Bangladesh; however one should point out that the abovelinked study measured bicarbonate using an ion-selective electrode rather than the normal enzymatic assay).
Derivation of the actual bicarbonate value
cHCO3-(P) includes ions of hydrogen carbonate, carbonate and carbamate in the plasma.
It is calculated from the Henderson Hasselbalch equation:
Well, its not completely like that. The pKa for carbonic acid is actually closer to 6.125 at body temperature (37°C), and this is assumed to be constant. Also the KH CO2 (Henry's constant) for CO2 is usually expressed in SI for the purposes of these calculations, and is therefore 0.23 mmol/L/kPa rather than 0.03mmol/L/mmHg.
The ABG machine takes this equation and rearranges it for the purpose of deriving an actual bicarbonate concentration from the pCO2 and pH measurements:
The reference manual for the local unit does not elaborate on how the constants were selected.
The handy online Henderson-Hasselbalch calculator available from Cornell University uses a slightly simpler equation:
HCO3- = 0.03 × pCO2 × 10(pH- 6.1)
The meaning of the actual bicarbonate value
In short, this is a parameter which can be used to assess the metabolic and respiratory causes of acid-base disturbances. However, it is an imperfect instrument. As the bicarbonate value changes with both respiratory and metabolic acid-base disturbances, it cannot be said to be an especially good measure of either.
There is also the standard bicarbonate, which is in essence a poor cousin of the standard base excess on the clinical battlefield, and therefore essentially obsolete. However, it is a more accurate representation of the "purely metabolic" component of acid-base balance. Often, in modern ABG machines the actual bicarbonate is usually calculated but not reported in the print-out.
The actual bicarbonate is useful when one is calculating a dose of bicarbonate to administer (though this can also be calculated from the standard bicarbonate value). In the absence of any respiratory acid-base disturbance (i.e. with a normal pCO2) it can be useful as a measure of metabolic acid base disturbance, but then it basically becomes "standard bicarbonate" anyway.
Additionally, the actual bicarbonate is what one should use when one is assessing the degree of compensation in acid-base disorders using the Boston bedside blood gas rules. As discussed elsewhere, and as illustrated by some extreme examples, using the standard bicarbonate value to estimate the effectiveness of compensation can lead one to inaccurate conclusions.
Actual bicarbonate has one other specific use, and that is in the calculation of the actual base excess, reported as cBase(B)c and discussed in another chapter.